The invention relates to a glass composition which comprises silicon dioxide, aluminum oxide, boric oxide, barium oxide and lead oxide.
The invention further relates to a fluorescent lamp comprising a hermetically sealed glass lamp-envelope, in which two electrodes connected to current supply wires are provided between which a discharge takes place during operation of the lamp, the lamp envelope being filled with a gas atmosphere and a metal which evaporates at least partly during operation of the lamp.
In a preferred embodiment of the invention the metal, which evaporates at least partly during operation of the lamp, comprises mercury.
The invention further relates to a cathode-ray tube comprising, in an evacuated envelope, a picture screen which has been provided on the inner side of a face plate which forms part of the wall of the envelope, which picture screen comprises luminescent material.
From Netherlands Patent Specification No. NL 55914 a glass composition is known, which apart from silicon dioxide, aluminum oxide, boric oxide, barium oxide and lead oxide also comprises alkali metal oxides and fluorine. The coefficient of expansion of this type of glass is, approximately, 90.multidot.10.sup.-7 K..sup.-1.
If the glass composition of the said Netherlands Patent Specification is used in a fluorescent lamp, the glass will be attacked by the metal vapour which is present in the lamp during operation, particularly if sodium vapour or mercury vapour is used. In addition, any phosphor present on the inside wall of the lamp envelope will be contaminated by mobile elements from the glass, due to the high temperature necessary for sealing and further shaping of the lamp. Both harmful effects can be attributed to the presence in the glass composition of alkali metal oxides, particularly sodium oxide. In order to obtain glass compositions having suitable working properties, the present state-of-the-art requires the presence of at least alkali metal oxides or lead oxide. A large quantity of lead oxide may, however, be disadvanteous, as under certain circumstances lead oxide is reduced to lead causing the glass to blacken. On the other hand, when replacing lead oxide by, for example, calcium oxide, the melting point of the glass becomes undesirably high.
When the above-described glass composition is used in a cathode-ray tube, particularly in a cathode-ray tube for projection television or for an oscilloscope, the high electric field-intensity will give rise to a high degree of contamination of the luminescent material by alkali metal oxides from the glass and to substantial discolouring of the glass by reduction of the lead oxide to lead.